On linux, we have the following dep chain:

aws-dsql-auth -> aws-c-http -> aws-c-io -> aws-lc

We only use aws-c-http for request signing, but we cannot make aws-c-io
conditionally required. aws-c-io only works with aws-lc on linux, which
means we land up in a situation where we have two libcryptos (postgres
wants openssl).

Our overall solution is to make fe-dsql-auth.o a self-contained
file (hiding the aws-dsql-auth dep chain) and explicitly exclude the
libs we don't want.

Doesn't handle occ yet

It doesn't work because we end up with a few unnamed syms.

This commit wires up the aws-c-common logging infra. You can log to
std{out,err} or a file, and set the level through env vars. The readme
has examples.

With this logging I realized that imds wasn't working due to a missing
http client. That's been fixed now.

The token gen infra (allocator, creds provider) are setup once,
globally. This lets us cache creds and do a fast-fail if there are no
creds.